The echoes of scattering feet are not an unusual sound for
radiologic technologists to hear. In fact, it is all too common for
those who work in the ER.

We have all been there. We help a patient sit upright, place a
cassette behind his or her back, walk 6 feet away, simply utter a word
and pandemonium occurs. People start diving behind walls or racing
across the hallway simply because we said the word "x-ray."

After witnessing this a few times, a couple of us R.T.s started to
wonder if we were in danger. Did these folks know something we
didn't? We harkened back to our training days for the resolution. A
refresher course in safety definitely was in order. So we dusted off our
books and grabbed a few additional ones to investigate the issue of
proximity to an x-ray source. We broke our task into three subtopics:
defining radiation, studying how radiation loses energy and determining
a relatively safe distance to stand from the x-ray source.

Radiation and Its Dangers

Electromagnetic radiation is photons of various energy levels.
X-radiation, specifically, is an ionizing type of radiation, that is,
radiation that potentially is harmful to living tissue because the
energy level is high enough to ionize atoms. Energy levels above 13.6 eV
have enough power to disrupt or ionize matter. In contrast to microwave
or radiofrequency wavelengths, x-radiation is strong enough to penetrate
and disrupt cells because it is capable of producing photons well above
13.6 eV. X-radiation is one of many sources of ionizing radiation.

Radiation is in our atmosphere, in our soil and in a few consumer
products, namely cathode-ray televisions and smoke detectors. We even
produce ionizing radiation ourselves. Our exposure ranges from 200 mRem
inhaled annually from radon in the atmosphere to 10mRem annually from
TVs and smoke detectors, according to Christensen's Physics of
Diagnostic Radiology.

The danger from ionizing radiation varies. The more you receive,
the more harmful it can be. Additionally, the longer you are exposed to
ionizing radiation, the more danger there is.

At 25,000 mRem, the human body shows signs of impairment. If a
person receives more than 300,000 mRem of ionizing radiation in one
occurrence, there is a 50 percent chance of mortality.

It's a Matter of Energy

Since medical x-rays are created by the radiographic unit, one
important factor in determining their energy level is the original
technical exposure factors used to create the image. All photons are not
equal; they have different energy levels. R.T.s adjust the amount or
quantity of photons created primarily through choosing mA and time
settings. The more mA applied, the more electrons are available for the
creation of photons; the more time you have to create photons, the more
of them there will be. Therefore, mA and time primarily determine
quantity.

The strength or quality of photons is associated with kVp, which
determines the penetrating ability of the photons. However, because the
x-ray beam is heterogeneous, there always will be relatively weak
photons no matter what kVp setting is used. When kVp is escalated, more
powerful photons are added, creating a beam with a mixture of weak,
medium and strong photons. Maximum energy is determined by the kVp used
to create the primary x-ray beam.

After these diverse photons depart from their source of production,
they encounter a number of impediments that significantly reduce their
overall effectiveness. Photons diverge as they leave the x-ray tube,
dispersing in random straight lines, and are absorbed by anything in
their path. As a result, the number of photons in a given area (photon
fluence) significantly decreases the farther away they are from the
source.

As photons pass through an object, including room air, they lose
power as they interact with that object's atoms. This interaction,
or attenuation, depends on the thickness and density of the object.
Relatively dense items such as bones attenuate more photons than less
dense tissues. Similarly, a thicker anatomical structure such as a femur attenuates more photons than a thinner structure such as a finger.

The combination of a heterogeneous primary beam and diverging and
attenuated photons results in intensity loss, which is the amount of
energy per area and per unit of time. The farther we stand from the
beam's source, the less powerful the beam. Using the inverse square
law, intensity is reduced by one quarter every time the distance is
doubled. For example, if you are 1 foot from the source in an area with
an intensity level of 10 R, moving an additional foot away reduces the
intensity to 2.5 R.

Exposure Levels Count

Determining how far from the source R.T.s should stand when
conducting a radiographic exam is complicated. The primary beam, leakage
from the x-ray tube and scatter radiation from the patient must be taken
into consideration. Additionally, what type of dose amount is being
considered--gonadal, organ, marrow? To keep this as simple as possible,
I will only reference the radiation that reaches the topical skin and
leave the other types of measurements for another time.

There are very few practices that potentially expose R.T.s and
others in the room to the primary beam. This danger could come from
holding the patient or improperly collimating and misdirecting a
horizontal tube. The three most common exams that require a horizontal
beam from a portable machine are the AP chest, lateral cross-table
C-spine and surgical cross-table lateral hip. R.T.s should adjust the
mobile equipment to ensure that the primary beam is never directed
toward the operator. The following examples list patient entrance skin
exposure doses at the range of recommended SID, from Hendee and
Ritenour's Medical Imaging Physics.

For an AP chest exam, with 90 kVP at 4 mAs, the exposure is 19.5 mR
at 6 feet SID and 4.8 mR at 12 feet SID.

For a lateral cross-table C-spine exam with 80 kVp at 32 mAs, the
exposure is 124.6 mR at 6 feet SID and 31.15 mR at 12 feet SID.

I used my medical facility's ER exposure chart to calculate
the exposure of a surgical cross-table lateral hip exam using 90 kVp at
80 mAs with an SID of 3 feet 6 inches. The exposure is 1,113 mR. At an
SID of 9 feet 6 inches, it's 142 mR.

Scatter radiation, particularly Compton scatter, causes great alarm
because it is the major source of exposure to the R.T. Compton scatter
occurs when a photon from the primary beam deflected from its primary
path.

The amount of scatter depends on the exposure technique used to
produce the image, field size, patient thickness, object density and
even room design. Important variables include the composition of the
room walls and how close they are because the electron continues to
bounce off objects until it is completely absorbed loses its energy.
Scattered photons are of different strengths, depending on their angle
of dispersion. The most energy is lost when a photon is scattered at a
180-degree angle and lessens it nears 0 degrees.

The following examples are based on scatter produced from a phantom
that is 9 inches thick. The loss of energy is from kVp only, not mA,
assuming all the photons are deflected at the same angle and none are
absorbed or otherwise alleviated. The numbers result in significantly
higher dose calculations than would occur in reality, and go beyond
worst-case scenario.

The data include the same three exams that were used for the
primary beam exposure and two more: the supine abdomen exam, which is
frequently ordered for inpatients, and the lateral spot L-spine exam,
which requires the most radiation to produce an image.

Using a 6 foot distance, the scatter from an AP chest exam at 90
degrees, according to Stoker's Introduction to Chemical Principles,
is 3.6 mR. For a lateral cross-table C-spine exam, it's 3.4 mR;
234.5 mR for a surgical cross-table lateral hip exam; 76 mR for a supine
abdomen exam and 351.8 mR for a lateral spot L-spine exam.

Now to the Question

Putting everything in perspective, keep in mind that there is no
escape from ionizing radiation. It occurs naturally from the sky and the
rocks and soil we walk on. We welcome it into our homes with open arms
when we use smoke detectors to protect our loved ones. Since we
can't avoid exposure to natural, background radiation, we must try
to limit, whenever possible, our occupational exposure.

In a real-life scenario, adhering to the ALARA (as low as
reasonably achievable) concept and practicing sound radiation safety
principles will lessen our exposure. If the R.T. stands at a 180-degree
angle from the patient, is 6 feet away from the x-ray source, wears the
appropriate protective apparel and is not in the beam's primary
path, the occupational exposure should be minimal.

The old adage that knowledge is power applies well here. Most
institutions have policies for staff technologists on radiation
protection, and are regulated by agencies such as the National Council
on Radiation Protection & Measurements. There is a need to follow
the cardinal principles of radiation safety, which include minimizing
the time you are exposed to ionizing radiation, maximizing the distance
from the source and maximizing shielding. When those practices are
followed, no one should feel the need to run for cover.

George Tolekidis, B.A., R.T.(R), works part time at Sutter Solano
Medical Center in Vallejo, Calif. He works full time at nearby Travis
Air Force Base where he is on active duty.

If you have a story idea for Your Turn, please contact ASRT Scanner
Editor D.D. Wolohan at dwolohan@asrt.org.

By George Tolekidis, B.A., R.T.(R), Contributing Writer

COPYRIGHT 2007 American Society of Radiologic Technologists
No portion of this article can be reproduced without the express written permission from the copyright holder.